Flexible κ4-PNN′O-tetradentate ligands: Synthesis, complexation and structural studies

Article abstract of DOI:10.1039/c0dt00200c

The three-step synthesis of new, air-stable, PNN′O-tetradentate ligands 3a·HH-3c·HH by Schiff base condensation of the 1°amines 2a-2c with 2-Ph₂PC₆H₄(CHO) in refluxing EtOH is described. The racemic ligand 3d·HH, isolated in 79% yield, was successfully prepared from 2-C₆H₄(OH){C(O) NHCH₂CH(Me)NH₂} 2d and 2-Ph₂PC ₆H₄(CHO) in absolute EtOH. Upon careful choice of metal precursor, ligands 3a·HH-3d·HH display various coordination modes. Reaction of 3a·HH with AuCl(tht) (1:1 molar ratio) affords AuCl(3a·HH), 4a, in which κ¹-P-complexation of the functionalised ligand is observed. In contrast, reaction of 3a·HH (or 3d·HH) with MCl₂(cod) (M = Pt, Pd) affords MCl ₂(3a·HH) (M = Pt, 5a; M = Pd, 5b) or MCl ₂(3d·HH) (M = Pt, 5c; M = Pd, 5d) in which ligand chelation is achieved using both P and imine N donor atoms. Moreover κ²-P, N-chelation was also observed when 3a·HH-3c·HH were separately allowed to react with [PdCl(η³-C₃H₅)] ₂ in CH₂Cl₂ affording new cationic η³-allyl complexes [Pd(η³-C₃H ₅)(3a·HH-3c·HH)]Cl, 6a-6c. Two neutral (methyl)chloropalladium(ii) complexes, 7a/7c, were isolated in high yields from 3a·HH or 3c·HH and Pd(CH₃)Cl(cod). Elimination of cod and single methyl protonation from Pt(CH₃)₂(cod) with 1 equiv. of 3a·HH, 3b·HH or 3d·HH in toluene, at room temperature, afforded square-planar complexes Pt(CH₃) (κ³-3a·H/3b·H/3d·H), 8a/8b/8d, containing monoanionic κ³-PNN′-tridentate ligands. The κ³-PNN′-tridentate mode was likewise observed for Pd(CH₃)(3a·H/3c·H), 10a/10c, upon treatment of a methanolic solution of Pd(CH₃)Cl(3a·HH/3c·HH) with ^tBuOK. Similarly the monohapto (allyl)Pd^II compounds Pd(CH₂CHCH₂)(3a·HH-3c·HH), 9a-9c, were obtained cleanly from 6a-6c and ^tBuOK via an η ³→η¹ allyl isomerisation. Both amide and phenolic protons in 5a-5d were smoothly deprotonated, with base, to give the κ⁴-PNN′O-tetradentate complexes 11a/11b and 11d/11e containing the dianionic ligands 3a^2-/3d^2- respectively. The Ni^II complexes 11c and 11f were synthesised directly from NiCl₂·6H₂O, 3a·HH (or 3d·HH) and ^tBuOK in CH₃OH. All new compounds were characterised by multinuclear NMR, FT-IR, mass spectrometry and microanalysis. Single crystal X-ray studies have been undertaken on the compounds 3a·HH, 3c·HH, 4a, 7c, 8a, 8b, 8d and 11a-11d. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00200c

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PAPER
www.rsc.org/dalton | Dalton Transactions
Flexible j4-PNN¢O-tetradentate ligands: synthesis, complexation
and structural studies†
Sean E. Durran, Mark R. J. Elsegood, Shelly R. Hammond and Martin B. Smith*
Received 24th March 2010, Accepted 12th May 2010
First published as an Advance Article on the web 24th June 2010
DOI: 10.1039/c0dt00200c
The three-step synthesis of new, air-stable, PNN¢O-tetradentate ligands 3a·HH–3c·HH by Schiff base
condensation of the 1◦ amines 2a–2c with 2-Ph2PC6H4(CHO) in refluxing EtOH is described. The
racemic ligand 3d·HH, isolated in 79% yield, was successfully prepared from
2-C6H4(OH){C(O)NHCH2CH(Me)NH2} 2d and 2-Ph2PC6H4(CHO) in absolute EtOH. Upon careful
choice of metal precursor, ligands 3a·HH–3d·HH display various coordination modes. Reaction of
1
3a·HH with AuCl(tht) (1 : 1 molar ratio) affords AuCl(3a·HH), 4a, in which k -P-complexation of the
functionalised ligand is observed. In contrast, reaction of 3a·HH (or 3d·HH) with MCl2(cod) (M = Pt,
Pd) affords MCl2(3a·HH) (M = Pt, 5a; M = Pd, 5b) or MCl2(3d·HH) (M = Pt, 5c; M = Pd, 5d) in
2
which ligand chelation is achieved using both P and imine N donor atoms. Moreover k -P,N-chelation
3
was also observed when 3a·HH–3c·HH were separately allowed to react with [PdCl(h -C3H5)]2 in
3
3
CH2Cl2 affording new cationic h -allyl complexes [Pd(h -C3H5)(3a·HH–3c·HH)]Cl, 6a–6c. Two neutral
(methyl)chloropalladium(II) complexes, 7a/7c, were isolated in high yields from 3a·HH or 3c·HH and
Pd(CH3)Cl(cod). Elimination of cod and single methyl protonation from Pt(CH3)2(cod) with 1 equiv. of
3a·HH, 3b·HH or 3d·HH in toluene, at room temperature, afforded square-planar complexes
3
3
Pt(CH3)(k -3a·H/3b·H/3d·H), 8a/8b/8d, containing monoanionic k -PNN¢-tridentate ligands. The
3
k -PNN¢-tridentate mode was likewise observed for Pd(CH3)(3a·H/3c·H), 10a/10c, upon treatment of
a methanolic solution of Pd(CH3)Cl(3a·HH/3c·HH) with tBuOK. Similarly the monohapto (allyl)PdII
t
=
compounds Pd(CH2CH CH2)(3a·HH–3c·HH), 9a–9c, were obtained cleanly from 6a–6c and BuOK
3
1
via an h →h allyl isomerisation. Both amide and phenolic protons in 5a–5d were smoothly
4
deprotonated, with base, to give the k -PNN¢O-tetradentate complexes 11a/11b and 11d/11e
containing the dianionic ligands 3a2-/3d2- respectively. The NiII complexes 11c and 11f were synthesised
directly from NiCl2·6H2O, 3a·HH (or 3d·HH) and tBuOK in CH3OH. All new compounds were
characterised by multinuclear NMR, FT–IR, mass spectrometry and microanalysis. Single crystal
X-ray studies have been undertaken on the compounds 3a·HH, 3c·HH, 4a, 7c, 8a, 8b, 8d and 11a–11d.
crystalline9 and phosphorescent properties10 and applications in
Introduction
catalysis (including asymmetric variations).11 Within this family of
ligands the most popular examples of donor set combinations are
thosecomprisingN2O212 andP2N213 atoms whereas unsymmetrical
systems e.g. PNN¢N¢¢ are either uncommon,14 or in the case of the
PNN¢O donor motif, extremely rare.15 Barandov and Abram16
recently described the synthesis of two new pentadentate Schiff
base ligands containing a rare PN2O2 donor set. Herein we describe
Significant developments in functionalised phosphine chemistry1
continue to play a crucial role in understanding how these
versatile ligands coordinate to metals, influence metal reactivity
(stereoelectronic properties) and find applications in, for example,
homogeneous catalysis. The marriage of two different donor
atoms, one a soft PIII centre and the other typically N2 or O,3
has led to a wealth of tertiary phosphines being studied of which
hemilabile ligands4 are a notable class. Whilst many functionalised
tertiary phosphines have been shown to act as bidentate ligands,5
few literature examples of tridentate PNO systems are known.6
In contrast, tetradentate ligands with two (or more) donor types,
including tetradentate p radical ligands,7 have attracted consid-
erable interest for their radioimaging/radiotherapeutic,8 liquid
4
the synthesis of a series of tetradentate k -PNN¢O ligands and a
stepwise survey of their coordination capabilities towards selected
transition-metal centres. Our studies have revealed these flexible
4
k -PNN¢O ligands adopt numerous ligation modes and afford
new examples of chelate stabilised amido2b and amido/phenoxo
metal(II) complexes. All new compounds have been structurally
verified both in solution and the solid-state.
Department of Chemistry, Loughborough University, Loughborough, UK
LE11 3TU. E-mail: m.b.smith@lboro.ac.uk; Fax: +44 1509 223925;
Tel: +44 1509 222553
Experimental
† Electronic supplementary information (ESI) available: Experimental
procedures for 1b, 1c, 2b and 2c; spectroscopic data for 1b, 1c, 2b and 2c
and additional X-ray figures. CCDC reference numbers 771445–771448,
771449–771452, 771453–771455. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c0dt00200c
Materials
All reactions were carried out under aerobic conditions, with
the exception of ligands 3a·HH–3d·HH whose syntheses were
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conducted under an atmosphere of dry, oxygen-free, nitro-
gen. All solvents were distilled prior to use. The com-
pounds 1a,17 2a,17 2d,18 2-Ph2PC6H4(CHO),19 AuCl(tht) (tht =
tetrahydrothiophene),20 MCl2(cod) (M = Pt, Pd; cod = cycloocta-
1,5-diene),21 Pd(CH3)Cl(cod)22 and Pt(CH3)2(cod)23 were all pre-
pared according to published procedures. All other reagents were
purchased from commercial suppliers.
r.t., the volume was reduced to ca. 10 ml under reduced pressure
and the solid collected by suction filtration. The solid was washed
with a small portion of EtOH and dried in vacuo. Yield: 0.498 g,
1
61%. Selected data: 31P: -8.1 ppm. H: 9.86 (s, 1H), 9.41 (s, 1H),
8.61 (d, 4JPH 4.0, 1H), 7.77-6.85 (m, 18H), 4.32 (s, 2H) ppm. FT–
IR: 3304, 1655 cm-1. EI–MS: m/z 438 [M+]. Anal. (%) Calcd.
for C27H23N2O2P: C, 73.95; H, 5.30; N, 6.39. Found: C, 73.39;
H, 5.25; N, 6.33. Compounds 3b·HH and 3c·HH were similarly
prepared in 65% and 76% yields respectively. Selected data for
Instrumentation
1
3b·HH: 31P: -11.4 ppm. H: 9.54 (s, 1H), 8.83 (s, 1H), 8.04-6.86
FT–IR spectra were recorded as KBr pellets over the range 4000-
400 cm-1 using a Perkin-Elmer system 2000 FT spectrometer.
(m, 18H), 4.64 (s, 2H), 4.28 (s, 2H) ppm. FT–IR: 3371, 3269 (NH,
OH), 1673 (CO amide I), 1534 (CO amide II) cm-1. EI–MS: m/z
452 [M+]. Anal. (%) Calcd. for C28H25N2O2P·0.5H2O: C, 72.86; H,
5.69; N, 6.07. Found: C, 72.69; H, 5.61; N, 5.83. Selected data for
1
1
The H NMR and 31P{ H} NMR spectra were recorded either
on Bruker AC250 or DPX-400 FT spectrometers with chemical
shifts (d) reported relative to external TMS or H3PO4. All NMR
spectra (250 or 400 MHz) were recorded in CDCl3 solutions unless
otherwise stated. Elemental analyses (Perkin-Elmer 2400 CHN
Elemental Analyzer) were performed by the Loughborough Uni-
versity Analytical Service within the Department of Chemistry.
1
4
3c·HH: 31P: -8.8 ppm. H: 9.39 (d, JPH 3.8, 1H), 8.58 (m, 1H),
7.77-6.77 (m, 18H), 5.83 (s, 1H), 4.25 (s, 2H) ppm. FT–IR: 3249,
3145, 1659, 1649 cm-1. EI–MS: m/z 438 [M+]. Anal. (%) Calcd.
for C27H23N2O2P·C2H5OH: C, 71.88; H, 6.28; N, 6.05. Found: C,
71.88; H, 5.84; N, 5.78.
=
Preparation of 1,2-(OH)C6H4{C(O)NHCH2CH(Me)N CH-
X-ray crystallography
C6H4PPh2} (3d·HH). A mixture of 2d (0.326 g, 1.678 mmol)
and 2-Ph2PC6H4(CHO) (0.505 g, 1.740 mmol) in absolute EtOH
(40 ml) was stirred for ca. 24 h. The solvent was evaporated to ca.
10 ml and 3d·HH isolated by suction filtration and dried in vacuo.
Suitable crystals were grown by vapour diffusion of Et2O into
either a CDCl3/CH2Cl2 (for 4a), CH2Cl2–CH3OH (for 7c·CH2Cl2)
or CDCl3 solution (for 8b·1.5C7H8, 11b·0.5CHCl3·0.5Et2O,
11c and 11d·CHCl3). Slow evaporation of an EtOH (for
3a·HH·EtOH), CHCl3 (for 3c·HH·0.5CHCl3) or CH2Cl2–Et2O
solution (for 11a·MeOH) afforded X-ray quality crystals. Suitable
crystals of 8a·CHCl3 (and 8d·2C7H8) were obtained from a C7H8
solution of 3a·HH (or 3d·HH) and Pt(CH3)2(cod). All measure-
ments were made on a Bruker AXS SMART 1000 CCD area-
detector diffractometer, at 150 K, using graphite-monochromated
Yield: 0.618 g, 79%. Selected data: 31P: -7.8 ppm. H: 12.56 (s,
1H), 8.53 (d, JPH 3.8, 1H), 7.75-6.71 (m, 19H), 3.70 (m, 1H),
1
4
3
3.53 (m, 1H), 3.24 (m, 1H), 0.93 (d, JHH 6.4, 3H) ppm. FT–IR:
3240, 1641, 1630 cm-1. ES–MS: m/z 467 [M+]. Anal. (%) Calcd.
for C29H27N2O2P: C, 74.65; H, 5.85; N, 6.01. Found: C, 74.92; H,
5.64; N, 6.34.
˚
=
Preparation of 1,2-(OH)C6H4{NHC(O)CH2N CHC6H4-
Mo-Ka radiation (l = 0.71073 A) and narrow frame exposures
(0.3◦) in w. Cell parameters were refined from the observed (w)
angles of all strong reflections in each data set. Intensities were
corrected semiempirically for absorption based on symmetry-
equivalent and repeated reflections. The structures were solved by
direct methods (Patterson synthesis for 7c·CH2Cl2 and 11d·CHCl3)
and refined on F2 values for all unique data by full-matrix
least-squares (Table 1). All non-hydrogen atoms were refined
anisotropically. Programs used were Bruker AXS SMART and
SAINT for diffractometer control and frame integration,24 Bruker
SHELXTL for structure solution, refinement and molecular
graphics,25 and local programs.
PPh2AuCl} (4a). AuCl(tht) (0.0056 g, 0.0175 mmol) was
dissolved in CH2Cl2 (10 ml) and 3a·HH (0.079 g, 0.0180 mmol)
added to afford a colourless solution. After stirring the solution
for 45 min, the volume was reduced to ca. 1–2 ml and diethyl
ether (20 ml) and petroleum ether (b.p 60–80 ◦C, 10 ml) added
to give 4a which was collected by filtration and dried in vacuo.
Yield: 0.107 g, 91%. Selected data: 31P: 29.2 ppm. 1H: 8.82 (s, 1H),
8.68 (s, 1H), 7.94 (s, 1H), 7.69-6.78 (m, 18H), 4.36 (s, 2H) ppm.
FT–IR: 3294, 1655 cm-1. ES–MS: m/z 635 [M–Cl]. Anal. (%)
Calcd. for C27H23AuClN2O2P: C, 48.33; H, 3.46; N, 4.18. Found:
C, 48.51; H, 3.54; N, 3.71.
For 3c·HH·0.5CHCl3, the CHCl3 molecule was disordered
over an inversion centre. For 8b·1.5C7H8, point atom modelling
was attempted for the C7H8 molecules but no suitable disorder
model could be established. The Platon Squeeze procedure
was therefore successfully applied.26 For 8d·2C7H8, one of the
C7H8 molecules was disordered and successfully modelled. In
11b·0.5CHCl3·0.5Et2O, the highly disordered Et2O molecule was
modelled using the Platon Squeeze procedure. For the chiral
structures the absolute structure parameters were: 4a, x = 0.555(6),
twinned by inversion; 11d, x = -0.025(8), single enantiomer, well
determined.
=
Preparation of 1,2-(OH)C6H4{NHC(O)CH2N CHC6H4-
PPh2PtCl2} (5a). To a CH2Cl2 (20 ml) solution of PtCl2(cod)
(0.090 g, 0.241 mmol) was added 3a·HH (0.104 g, 0.237 mmol)
to give a yellow solution. After stirring the solution for 15 min,
the volume was reduced under vacuum to ca. 1–2 ml and
addition of diethyl ether (25 ml) afforded a yellow solid. The
solid was collected by suction filtration and dried in vacuo.
Yield: 0.150 g, 89%. Compound 5b (91%) was similarly
prepared from PdCl2(cod) and 3a·HH. Selected data for 5a:
31P (CDCl3/CH3OH): 10.7 ppm, JPtP 3492 Hz; 3.0 ppm, JPtP
3324 Hz. FT–IR: 3258, 1670, 1632 cm-1. ES–MS: m/z 669 [M–Cl].
Anal. (%) Calcd. for C27H23N2O2PPtCl2: C, 46.03; H, 3.30; N,
3.98. Found: C, 45.95; H, 3.28; N, 3.43. Selected data for 5b: 31P
1
1
=
Preparation of 1,2-(OH)C6H4{NHC(O)CH2N CHC6H4PPh2}
(3a·HH). A suspension of 2a (0.307 g, 1.848 mmol) and 2-
Ph2PC6H4(CHO) (0.556 g, 1.915 mmol) in absolute EtOH (30 ml)
was refluxed, under a N2 atmosphere, for ca. 4 h. After cooling to
1
(CDCl3/CH3OH): 38.2 ppm. H: 10.30 (s, 1H), 10.01 (s, 1H),
8.74 (s, 1H), 8.12-6.79 (m, 18H), 5.37 (s, 2H) ppm. FT–IR: 3324,
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3258, 1698, 1645 cm-1. MALDI–MS: m/z 580 [M–Cl]. Anal.
(%) Calcd. for C27H23N2O2PPdCl2: C, 52.66; H, 3.77; N, 4.55.
Found: C, 52.54; H, 3.72; N, 4.33. Compounds 5c (99%) and 5d
(92%) were similarly prepared from 3d·HH and the appropriate
MCl2(cod). Selected data for 5c: 31P [CDCl3/(CD3)2SO]: 6.1 ppm,
=
Preparation
of
1,2-(OH)C6H4{NC(O)CH2N CHC6H4-
PPh2Pt(CH3)} (8a). To the solids Pt(CH3)2(cod) (0.040 g,
0.120 mmol) and 3a·HH (0.053 g, 0.121 mmol) was added toluene
(2 ml) to afford a yellow solution. After standing for 2 d, a
yellow crystalline solid formed which was collected and dried.
1JPtP 3799 Hz. H: 12.18 (s, 1H), 8.41 (s, 1H), 8.37 (d, JPH 7.3,
1H), 7.70-6.66 (m, 18H), 5.64 (m, 1H), 4.05 (m, 1H), 3.16 (m, 1H),
0.78 (d, 3JHH 6.8, 3H) ppm. FT–IR: 3295, 1640 cm-1. ES–MS: m/z
697 [M–Cl]. Anal. (%) Calcd. for C29H27N2O2PPtCl2: C, 47.55; H,
3.72; N, 3.83. Found: C, 47.78; H, 3.78; N, 4.21. Selected data
1
4
3
Yield: 0.072 g, 92%. The k -PNN¢-tridentate complexes 8b (62%)
and 8d (84%) were prepared similarly. Selected data for 8a: 31P:
13.7 ppm, JPtP 3865 Hz. 1H: 8.43 (3JPtH 43.8, 1H), 7.57-6.72
1
(m, 18H), 4.84 (s, 2H), 0.00 (2JPtH 72.8, 3H) ppm. FT–IR: 2926,
1636, 1605, 1574 cm-1. ES–MS: m/z 646 [M+]. Anal. (%) Calcd.
for C28H25N2O2PPt·CHCl3: C, 45.42; H, 3.42; N, 3.65. Found:
C, 45.46; H, 3.27; N, 3.43. Selected data for 8b: 31P: 13.6 ppm,
1JPtP 3809 Hz. 1H: 8.63 (3JPtH 43.2, 1H), 7.71-7.06 (m, 18H),
1
for 5d: 31P [CDCl3/(CD3)2SO]: 32.9 ppm. H: 12.30 (s, 1H), 8.50
4
(d, JPH 6.2, 1H), 8.23-6.74 (m, 19H), 5.59 (m, 1H), 4.15 (m,
3
1H), 3.18 (m, 1H), 0.69 (d, JHH 6.8, 3H) ppm. FT–IR: 3269,
1639 cm-1. ES–MS: m/z 571 [M–2Cl–H+]. Anal. (%) Calcd. for
C29H27N2O2PPdCl2: C, 54.09; H, 4.24; N, 4.35. Found: C, 53.86;
H, 4.28; N, 4.33.
2
2
4.98 (s, 2H), 4.87 (d, JHH 11.0, 1H), 4.47 (d, JHH 11.0, 1H),
3.99 (br, 1H), 0.00 (2JPtH 72.0, 3H) ppm. FT–IR: 3384, 3292,
1639, 1602, 1585, 1570 cm-1. ES–MS: m/z 662 [M+]. Anal. (%)
Calcd. for C29H27N2O2PPt: C, 52.64; H, 4.12; N, 4.24. Found:
C, 52.58; H, 4.08; N, 3.81. Selected data for 8d: 31P: 14.4 ppm,
=
Preparation of [1,2-(OH)C6H4{NHC(O)CH2N CHC6H4-
3
PPh2Pd(g3-C3H5)}]Cl (6a). To a solution of [PdCl(h -C3H5)]2
1JPtP 3834 Hz. H: 12.60 (s, 1H), 8.62 (3JPtH 42.4, 1H), 8.35 (dd,
1
(0.037 g, 0.101 mmol) in CH2Cl2 (5 ml) was added 3a·HH
(0.091 g, 0.208 mmol) to afford a yellow solution. After stirring
for 1 h the solution was concentrated under reduced pressure to
ca. 1–2 ml and diethyl ether (10 ml) added. The yellow solid was
collected and dried in vacuo. Yield: 0.107 g, 85%. Compounds 6b
(96%) and 6c (84%) were prepared in a related manner. Selected
1H), 7.62-6.49 (m, 17H), 4.15 (ddd, 1H), 4.03 (m, 1H), 3.64 (dd,
1H), 1.33 (d, JHH 6.8, 3H), 0.01 (2JPtH 75.2, 3H) ppm. FT–IR:
3
1618, 1555 cm-1. ES–MS: m/z 676 [M+]. Anal. (%) Calcd. for
C30H29N2O2PPt·2C7H8: C, 61.45; H, 5.29; N, 3.26. Found: C,
61.57; H, 5.25; N, 3.52.
data for 6a: 31P: 24.3 ppm. H: 11.66 (s, 1H), 9.20 (s, 1H), 8.74
Preparation
of
=
1
=
1,2-(OH)C6H4{NC(O)CH2N CHC6H4-
1
(s, 1H), 7.84-6.85 (m, 18H), 5.83 (q, 1H), 5.41 (s, 2H), 4.00 (br),
3.07 (br) ppm. FT–IR: 3214, 3145, 1654 cm-1. ES–MS: m/z 585
[M–Cl]. Anal. (%) Calcd. for C30H28N2O2PPdCl: C, 57.98; H,
4.55; N, 4.51. Found: C, 57.46; H, 4.56; N, 4.52. Selected data
PPh2Pd(g -CH2CH CH2)} (9a). To a CH3OH (2 ml) solution
of 6a (0.051 g, 0.0821 mmol) was added tBuOK (0.012 g,
0.107 mmol) with immediate formation of a yellow solid. The
suspension was stirred for 30 min, and the solid 9a collected
1
1
for 6b: 31P: 24.2 ppm. H: 10.31 (s, 1H), 8.84 (s, 1H), 7.85-7.12
by suction filtration and dried. Yield: 0.041 g, 85%. The h -
(m, 19H), 5.82 (q, 1H), 5.44 (s, 2H), 4.61 (s, 2H), 4.06 (br), 3.03
(br) ppm. FT–IR: 3394, 1684 cm-1. ES–MS: m/z 599 [M–Cl].
Anal. (%) Calcd. for C31H30N2O2PPdCl·0.5H2O: C, 57.77; H,
4.86; N, 4.35. Found: C, 57.64; H, 4.84; N, 4.23. Selected data for
allylpalladium(II) complexes 9b (51%) and 9c (51%) were prepared
1
similarly. Selected data for 9a: 31P: 32.4 ppm. H: 8.33 (s, 1H),
=
8.27 (s, 1H), 7.64-6.85 (m, 18H), 5.39 (m, 1H, CH), 4.72 (s,
=
=
2H), 4.40 (d, 1H, CH2), 4.16 (d, 1H, CH2), 1.83 (m, 2H,
–CH2Pd) ppm. FT–IR: 3268, 1642, 1605, 1556 cm-1. ES–MS:
m/z 585 [M+]. Anal. (%) Calcd. for C30H27N2O2PPd: C, 61.59; H,
4.66; N, 4.79. Found: C, 61.46; H, 4.40; N, 4.45. Selected data
1
6c: 31P: 24.2 ppm. H: 10.56 (s, 1H), 8.57 (s, 1H), 7.82-6.74 (m,
19H), 5.70 (q, 1H), 5.09 (s, 2H), 3.90 (br), 2.90 (br) ppm. FT–IR:
3240, 3196, 3141, 1674 cm-1. ES–MS: m/z 585 [M–Cl]. Anal. (%)
Calcd. for C30H28N2O2PPdCl·1.5H2O: C, 55.56; H, 4.83; N, 4.32.
Found: C, 55.53; H, 4.68; N, 4.03.
for 9b: 31P: 32.6 ppm. H: 8.29 (s, 1H), 7.67-7.08 (m, 18H), 5.25
1
=
(m, 1H, CH), 4.73 (m, 2H), 4.64 (m, 2H), 4.58 (s, 1H), 4.18 (d,
=
=
1H, CH2), 3.74 (d, 1H, CH2), 1.60 (m, 2H, –CH2Pd) ppm.
FT–IR: 1648 (CO amide I), 1556 (CO amide II) cm-1. ES–MS:
m/z 599 [M+]. Anal. (%) Calcd. for C31H29N2O2PPd·1.5H2O: C,
59.47; H, 5.16; N, 4.48. Found: C, 59.29; H, 4.71; N, 4.54. Selected
data for 9c: 31P: 31.8 ppm. 1H: 8.37 (s, 1H), 8.33 (s, 1H), 7.67-6.75
=
Preparation of 1,2-(OH)C6H4{NHC(O)CH2N CHC6H4-
PPh2Pd(CH3)Cl} (7a). To
a
solution of Pd(CH3)Cl(cod)
(0.050 g, 0.189 mmol) in CH2Cl2 (10 ml) was added 3a·HH
(0.086 g, 0.196 mmol) to give an initial pale yellow solution
whereupon, after a few minutes, a colourless solid 7a deposited.
After stirring for 25 min, the volume was concentrated under
reduced pressure to ca. 2 ml and diethyl ether (20 ml) added.
The solid was collected and dried in vacuo. Yield: 0.097 g, 87%.
Compound 7c (91%) was likewise prepared. Selected data for 7a:
=
=
(m, 18H), 5.40 (m, 1H, CH), 4.62 (s, 2H), 4.20 (m, 1H, CH2),
=
3.86 (m, 1H, CH2), 1.70 (m, 2H, –CH2Pd) ppm. FT–IR: 3211,
1646, 1558 cm-1. ES–MS: m/z 585 [M+]. Anal. (%) Calcd. for
C30H27N2O2PPd: C, 61.59; H, 4.66; N, 4.79. Found: C, 60.90; H,
4.43; N, 4.44.
31P [CDCl3/CH3OH/(CH3)2SO]: 39.3 ppm. H [(CD3)2SO]: 9.68
1
=
1,2-(OH)C6H4{NC(O)CH2N CHC6H4-
(s, 1H), 9.32 (s, 1H), 8.37 (s, 1H), 7.66-6.54 (m, 18H), 4.85 (s,
Preparation
of
3
2H), 0.00 (d, JPH 3.2, 3H) ppm. FT–IR: 3242, 1650, 1628 cm-1.
PPh2Pd(CH3)} (10a). To a CH3OH (2 ml) solution of 7a
t
ES–MS: m/z 559 [M–Cl]. Anal. (%) Calcd. for C28H26N2O2PPdCl:
C, 56.48; H, 4.41; N, 4.71. Found: C, 56.14; H, 4.48; N, 4.68.
Selected data for 7c: 31P: 38.0 ppm. 1H: 10.29 (s, 1H), 8.25 (s, 1H),
(0.050 g, 0.084 mmol) was added BuOK (0.012 g, 0.107 mmol)
with immediate formation of a pale pink solid 10a. The mixture
was stirred for 40 min and the solid isolated by suction filtration
and dried in vacuo. Yield: 0.045 g, 96%. The methylpalladium(II)
complex 10c (74%) was similarly prepared. Selected data for
10a: 31P: 36.2 ppm. 1H: 8.36 (s, 1H), 8.29 (s, 1H), 7.68-6.82
3
7.53-6.71 (m, 18H), 6.39 (s, 1H), 4.88 (s, 2H), 0.53 (d, JPH 4.0,
3H) ppm. FT–IR: 3245, 3204, 3153, 1656, 1608 cm-1. ES–MS: m/z
559 [M–Cl]. Anal. (%) Calcd. for C28H26N2O2PPd·0.75CH2Cl2: C,
52.39; H, 4.21; N, 4.25. Found: C, 52.38; H, 4.24; N, 4.03.
3
(m, 18H), 4.77 (s, 2H), 0.00 (d, JPH 3.2, 3H) ppm. FT–IR:
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2945, 1651, 1594, 1567 cm-1. ES–MS: m/z 559 [M+]. Anal. (%)
Calcd. for C28H25N2O2PPd: C, 60.16; H, 4.52; N, 5.01. Found: C,
1
59.66; H, 4.20; N, 4.89. Selected data for 10c: 31P: 34.7 ppm. H
[CDCl3/(CD3)2SO]: 8.43 (s, 1H), 8.30 (s, 1H), 7.77-6.81 (m, 18H),
4.76 (s, 2H), 0.00 (d, 3JPH 3.2, 3H) ppm. FT–IR: 3384, 3212, 2949,
1646, 1555 cm-1. ES–MS: m/z 559 [M+]. Anal. (%) Calcd. for
C28H25N2O2PPd·H2O: C, 58.29; H, 4.73; N, 4.86. Found: C, 58.27;
H, 4.20; N, 4.79.
Scheme 1
=
Preparation of 1,2-(O)C6H4{NC(O)CH2N CHC6H4PPh2Pt}
the synthesis of ligands with N2O2
and N3O27b donor sets.
17,27a
(11a).
Method 1. A CH3OH (3 ml) solution of 5a (0.101 g, 0.143 mmol)
was treated with BuOK (0.044 g, 0.392 mmol). The orange/red
Reaction of the aminoalcohols RC6H4NH2 (R = 2-OH, 4-OH
or 2-CH2OH) with carbobenzyloxyglycine and a slight excess of
DCC in THF for 4 h gave 1a–1c in high yields. Treatment of
1a–1c with Pd/C (10%) and cyclohexene in refluxing EtOH gave,
after filtration and evaporation of the solvent, the 1◦ amines 2a–
2c (characterising data for 1b, 1c, 2b and 2c provided in the ESI).
Using a well established route14d,14e,16,28,29e for synthesising phosphi-
noimines, reaction of 2a–2c with 2-Ph2PC6H4(CHO) in refluxing
EtOH gave, after workup, the tetradentate ligands 3a·HH–3c·HH
in good yields (61–76%). This synthetic approach was also used
to prepare 3d·HH from 2-C6H4(OH){C(O)NHCH2CH(Me)NH2}
2d18 and 2-Ph2PC6H4(CHO) in EtOH at ambient temperature
(Scheme 2). Compounds 3a·HH–3d·HH were obtained as off-
white solids, soluble in a range of common organic solvents.
The spectroscopic data confirm Schiff base condensation, with
t
suspension was stirred for 40 min, filtered and washed with a
small portion of CH3OH. Yield: 0.079 g, 88%. Compound 11b was
prepared similarly in quantitative yield whereas 11c was prepared
(in 91% yield) from NiCl2·6H2O, tBuOK and 3a·HH. Selected data
for 11a: 31P: 8.0 ppm, 1JPtP 3371 Hz. 1H: 8.38 (3JPtH 99.6, 1H), 8.34
(d, 1H), 7.80-6.63 (m, 17H), 5.13 ppm (3JPtH 15.8, 2H). FT–IR:
1628 (CO), 1609 cm-1. ES–MS: m/z 632 [M+]. Anal. (%) Calcd.
for C27H21N2O2PPt: C, 51.35; H, 3.36; N, 4.44. Found: C, 50.82;
H, 3.17; N, 4.78. Selected data for 11b: 31P: 21.3 ppm. 1H: 8.18 (d,
1H), 8.05 (s, 1H), 7.70-6.68 (m, 17H), 5.03 ppm (s, 2H). FT–IR:
1642 (CO), 1616 cm-1. ES–MS: m/z 543 [M+]. Anal. (%) Calcd.
for C27H21N2O2PPd: C, 59.73; H, 3.91; N, 5.16. Found: C, 59.53;
H, 3.83; N, 5.85. Selected data for 11c: 31P: 19.7 ppm. 1H: 8.17 (d,
1H), 8.03 (s, 1H), 7.74-6.46 (m, 17H), 4.59 ppm (s, 2H). FT–IR:
1634 (CO), 1608 cm-1. ES–MS: m/z 495 [M+]. Anal. (%) Calcd.
for C27H21N2O2PNi·CH2Cl2: C, 57.97; H, 4.00; N, 4.83. Found: C,
57.84; H, 3.80; N, 4.90.
formation of a CH N bond, since new 31P signals were observed
=
at d(P) -8 ppm [cf. d(P) -11.2 ppm for 2-Ph2PC6H4(CHO)].
Furthermore the 1H NMR spectra, in all four cases, show a distinct
=
CH N resonance in the range d(H) 8.58–8.83 ppm and a singlet in
Method 2. To a CH2Cl2 (5 ml) solution of Pd(OAc)2 (0.025 g,
0.111 mmol) was added 3a·HH (0.050 g, 0.114 mmol) to give a
deep red solution. The solution was stirred for 45 min, the volume
reduced to ca. 1–2 ml and addition of diethyl ether (20 ml) gave
11b which was collected by suction filtration and dried in vacuo.
Yield: 0.056 g, 93%.
the region d(H) 4.25–4.32 ppm for the methylene group (CH2N).
Compounds 3a·HH–3d·HH adopt an E- (anti-) configuration as
previously observed for other phosphinoimine ligand systems14a
and based on single crystal X-ray studies for 3a·HH and 3c·HH
(vide infra).
The complexes 11d–11f (78, 83 and 80% yields respectively)
were prepared using the same procedure as for 11a–11c. Selected
1
1
data for 11d: 31P: 10.4 ppm, JPtP 3378 Hz. H: 8.58 (3JPtH 95.2,
1H), 8.32 (dd, 1H), 7.73-6.48 (m, 17H), 4.11 (m, 1H), 3.89 (ddd,
3
1H), 3.60 (dd, 1H), 1.39 (d, JHH 6.6, 3H) ppm. FT–IR: 1598,
1566, 1526 cm-1. ES–MS: m/z 660 [M+]. Anal. (%) Calcd. for
C29H25N2O2PPt·CHCl3: C, 46.25; H, 3.37; N, 3.60. Found: C,
Scheme 2
1
46.29; H, 3.32; N, 3.53. Selected data for 11e: 31P: 24.3 ppm. H:
8.27 (s, 1H), 8.24 (dd, 1H), 7.70-6.44 (m, 17H), 4.07 (m, 1H),
3.84 (ddd, 1H), 3.51 (dd, 1H), 1.32 (d, 3JHH 6.5, 3H) ppm. FT–IR:
1595, 1563, 1525 cm-1. ES–MS: m/z 571 [M+]. Anal. (%) Calcd. for
C29H25N2O2PPd: C, 61.00; H, 4.42; N, 4.91. Found: C, 60.36; H,
4.45; N, 4.65. Selected data for 11f: 31P: 22.7 ppm. 1H: 8.13 (m, 2H),
7.83-6.09 (m, 17H), 3.84 (m, 1H), 3.72 (ddd, 1H), 3.39 (dd, 1H),
1.51 (d, 3JHH 6.4, 3H) ppm. FT–IR: 1597, 1569, 1522 cm-1. FAB–
MS: m/z 523 [M+]. Anal. (%) Calcd. for C29H25N2O2PNi·CHCl3:
C, 56.07; H, 4.09; N, 4.36. Found: C, 55.51; H, 4.01; N, 4.12.
The X-ray structures of 3a·HH·EtOH [Fig. 1(a)] and
3c·HH·0.5CHCl3 [Fig. 1(b)] have been determined. In both 3a·HH
and 3c·HH the geometry around P(1) is distorted tetrahedral
with C–P–C angles in the range 101.03(8)–104.08(8)◦. The most
distinct structural difference between these isomeric ligands is
the relative orientation of the Ph2P– unit with respect to the
N(1)/N(2)/O(2) donor atoms. In 3a·HH the Ph2P– group points
away from N(1)/N(2)/O(2) yet in 3c·HH this group faces both
the N(1)/N(2) donor atoms. Free rotation about C(7)–C(aryl) is
plausible thus predisposing all four donor atoms (for 3a·HH) in
the same plane when bound in a tetradentate fashion. Various
H-bonding interactions in 3a·HH and 3c·HH exist including,
common to both structures, an intramolecular N(1) ◊ ◊ ◊ H(2)–N(2◦)
Results and discussion
Ligand syntheses
˚
The new unsymmetric ligands 3a·HH–3c·HH were synthesised
from 2a–2c (Scheme 1) using a previous literature method for
H-bond [N(1) ◊ ◊ ◊ N(2) 2.638(2) A, N(1) ◊ ◊ ◊ H(2)–N(2) 117.0(17)◦
˚
for 3a·HH; N(1) ◊ ◊ ◊ N(2) 2.688(2) A, N(1) ◊ ◊ ◊ H(2)–N(2) 116.6(16)
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Scheme 3
Fig. 2 Molecular structure of 4a. All hydrogen atoms except those on
N(2) and O(2) are removed for clarity.
Fig.
1
Molecular structures of (a) 3a·HH·EtOH and (b)
3c·HH·0.5CHCl3. The disordered CHCl3 and all hydrogen atoms
except those on N(2), O(2), O(3) and O(3A) are removed for clarity.
Symmetry operator A = -x + 1, y + 1/2, -z + 5/2.
[179.61(6)◦] parameters consistent with a near linear geometry
around Au(I). Upon coordination, the conformation of 3a·HH,
with respect to the NN¢O- group, remains unaltered. Furthermore
there persists a strong intramolecular N(1) ◊ ◊ ◊ H◦(2)–N(2) H-bond
for 3c·HH]. Furthermore, additional intermolecular H-bonding
˚
[N(1) ◊ ◊ ◊ N(2) 2.592(5) A, N(1) ◊ ◊ ◊ H(2)–N(2) 116 ] and neighbour-
˚
to an EtOH solvate [O(1) ◊ ◊ ◊ O(3A) 2.6846(19) A, O(1) ◊ ◊ ◊ H(3B)–
◦
ing molecules form chains (see ESI) along the a direction through
˚
O(3A) 172(3) and O(2) ◊ ◊ ◊ O(3) 2.598(2) A, O(2)–H(2A) ◊ ◊ ◊ O(3)
˚
167(3)◦] is evident in 3a·HH. Compound 3c·HH forms chains
O–H ◊ ◊ ◊ O intermolecular H-bonds [O(2) ◊ ◊ ◊ O(1A) 2.661(4) A,
O(2)–H(2A) ◊ ◊ ◊ O(1A) 143◦].
along the c direction via intermolecular H-bonding [O(2) ◊ ◊ ◊ O(1A)
◦
Phosphinoimines are well known to form late transition-metal
chelate complexes using both P and imine N-donor atoms.29
Furthermore PdII iminophosphine complexes have been used
in a range of catalytic applications highlighting the versatility
of these ligand systems.6f,30 In order to probe the coordination
behaviour of ligands 3a·HH–3d·HH treatment of MCl2(cod)
(M = Pt, Pd) with one equiv. of 3a·HH (or 3d·HH) afforded
the chelate complexes 5a, 5b (or 5c, 5d) (Scheme’s 3 & 4).
˚
2.694(2) A, O(2)–H(2A) ◊ ◊ ◊ O(1A) 158(3) ] between adjacent
molecules (see ESI† for further details).
Coordination studies
The reactivity of 3a·HH–3d·HH towards linear and square-planar
metal centres was explored in order to evaluate their flexibility and
1
coordination potential. Classical k -P-coordination was achieved
3
upon stoichiometric reaction of 3a·HH with AuCl(tht) in CH2Cl2
affording 4a in 91% yield (Scheme 3). The downfield shift of d(P)
29.2 ppm [in addition to a minor (ca. 5%) species identified as
AuCl{2-Ph2PC6H4(CHO)}, d(P) 32.2 ppm, presumably arising
from hydrolysis of the imine bond] clearly support P-coordination
of 3a·HH.
Mononuclear cationic [Pd(h -C3H5)(3a·HH–3c·HH)]Cl (6a–6c)
and neutral Pd(CH3)Cl(3a·HH/3c·HH) (7a,7c) complexes were
3
readily prepared from [PdCl(h -C3H5)]2 and Pd(CH3)Cl(cod)
2
(Scheme’s 3 & 5). In all complexes, k -P,N-chelation was deduced
from downfield phosphorus chemical shifts [d(P) 38.2 ppm (for
5b); ca. 24 ppm (for 6a–6c); ca. 38 ppm (for 7a, 7c)]. For 5a,
1
The X-ray structure of 4a (Fig. 2, Table 2) shows typi-
in CDCl3/CH3OH solution, two species at d(P) 10.7 ppm, JPtP
1
˚
˚
cal Au–P [2.2283(10) A], Au–Cl [2.2714(12) A] and Cl–Au–P
3492 Hz and 3.0 ppm, JPtP 3324 Hz were observed, tentatively
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Scheme 4
Scheme 5
assigned neutral and cationic structures, the later presumably
involving NH/OH coordination.
The X-ray structure of 7c·CH2Cl2 (Fig. 3, Table 2) shows the
2
phosphinoimine k -P,N-donor atoms to chelate the palladium(II)
centre with six-membered ring formation. The methyl group lies
trans to N with Pd–P, Pd–N, Pd–C and Pd–Cl bond lengths
similar to those in previous reported compounds.6j In 7c·CH2Cl2
the “Pd(CH3)Cl” fragment is hinged about P(1) ◊ ◊ ◊ N(1)/C(7)
˚
with P(1)–C(1)–C(6)–C(7) lying essentially flat (within 0.032 A).
The Pd(1) metal atom resides out of the basal plane of the
˚
P(1), N(1), Cl(1) and C(28) donor substituents by 0.027 A. An
˚
intermolecular O(1¢) ◊ ◊ ◊ H(2A)–O(2) [O(1¢) ◊ ◊ ◊ O(2) 2.696(3) A and
O(1¢) ◊ ◊ ◊ H(2A)–O(2) 178(4)◦; symmetry operator ¢ = x, –y + 1/2, z
Fig. 3 Molecular structure of 7c. The CH2Cl2 solvent of crystallisation
and all hydrogen atoms except on N(2) are removed for clarity.
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–1/2] H-bond links molecules into chains along the c direction (see
ESI†) and there also exists an intramolecular N(2)–H(2) ◊ ◊ ◊◦Cl(1)
˚
[N(2) ◊ ◊ ◊ Cl(1) 3.131(2) A and N(2)–H(2) ◊ ◊ ◊ Cl(1) 166(3) ] H-
bond. From the solid state structure it is possible to envisage how
3c·HH could further function as a tridentate ligand.
In order to verify whether ligands 3a·HH–3d·HH could function
3
as k -PNN¢-tridentate ligands it was necessary to find a synthetic
method which would permit single deprotonation of the NH amide
group. Klein et al.31 demonstrated the secondary amide phosphine
2-Ph2PC6H4(NHR) smoothly reacts with Co(CH3){P(CH3)3}4 to
afford a five-coordinate CoI complex accompanied by CH4 elimi-
nation. When Pt(CH3)2(cod) was employed instead, we reasoned
the P,Nimine group would displace the cod ligand and position
the coordinated ligand for methyl protonation/elimination by the
NH amide hydrogen (the OH group could function similarly).
Satisfyingly, in toluene this reaction proceeds cleanly to give 8a
(and 8b, 8d) in high yields as yellow, air-stable solids (Scheme’s
4 & 6) which were fully characterised by NMR, FT–IR, ES–MS
Fig. 6 Molecular structure of 8d. The C7H8 solvent is removed for clarity.
1
and microanalyses. In the 31P{ H} NMR spectra of 8a, 8b and
8d, clean 31P resonances were observed around d(P) 13 ppm [1JPtP
3800–3870 Hz]. The 1H NMR spectra clearly show retention of one
methyl group [d(H) ca. 0.00 ppm, 2JPtH ca. 73 Hz] and was further
corroborated by single crystal X-ray structure determinations of
8a (Fig. 4), 8b (Fig. 5) and 8d (Fig. 6).
Scheme 6
geometries around N(1) and N(2) can be inferred from bond
ꢀ
angles (ca.
360◦) at both nitrogen centres in accord with
amide functional groups. In 8a, the Pt(1)–N(1)–C(7)–C(6)–C(1)
ring is essentially flat (within 0.09 A) with P(1) lying 0.39 A
˚
˚
Fig. 4 Molecular structure of 8a showing the intramolecular O–H ◊ ◊ ◊ O
H-bond. The CHCl3 solvent of crystallisation is removed for clarity.
out of this plane whereas in 8b the N(1)–C(7)–C(6)–C(1)–P(1)
ring is essentially flat (within 0.07 A) with Pt(1) lying 0.37 A
˚
˚
out of this plane. For the five-membered ring in 8a, N(1) lies
˚
0.38 A out of the plane with respect to Pt(1)–N(2)–C(9)–C(8)
˚
which is essentially flat (within 0.004 A) and this feature remains
for 8b. For 8d, the N(2)–Pt(1)–N(1)–C(8) is co-planar (within
˚
˚
0.006 A) with C(10) out of this plane by 0.68 A resulting
in an envelope conformation. The two compounds 8a and 8b,
differing only by an extra methylene group, give rise to disparate
intramolecular and intermolecular O ◊ ◊ ◊ H–O H-bonding motifs.
In 8a and 8d there is an intramolecular O(1) ◊ ◊ ◊ H(2)–O(2) H-
◦
˚
bond [O(1) ◊ ◊ ◊ O(2) 2.543(5) A, O(1) ◊ ◊ ◊ H(2)–O(2)◦ 151 for 8a;
˚
O(1) ◊ ◊ ◊ O(2) 2.509(5) A, O(1) ◊ ◊ ◊ H(2)–O(2) 160(6) for 8d]. For
Fig. 5 Molecular structure of 8b showing the H-bonded dimer pair
formation. The C7H8 solvent of crystallisation is removed for clarity.
8b, dimer pairs are formed through intermolecular O ◊ ◊ ◊ H–O
◦
˚
H-bonding [O(1A) ◊ ◊ ◊ O(2) 2.758(5) A, O(1A) ◊ ◊ ◊ H(2)–O(2) 127 ;
The complexes 8a, 8b and 8d display an essentially square-
planar environment around the platinum centre with bond angles
in the range 80.31(13)–95.45(9)◦ (Table 2). The Pt–N(1) and
Pt–N(2) bond lengths vary slightly whilst the Pt–P and Pt–
C bond distances are similar.14a Furthermore, trigonal planar
symmetry operator A = -x+1, -y+1, -z+1] generating an R2 (16)
2
ring motif. The effect of amide deprotonation and complexation
has clearly reduced the potential for any further H-bonding.
Two further studies were undertaken regarding the formation
1
of 8a and 8b from Pt(CH3)2(cod). Firstly, in solution by 31P{ H}
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NMR spectroscopy, could we possibly observe the intermediate
formation of Pt(CH3)2(3a·HH) 8a¢ and, secondly, could a second
methane elimination be thermally induced? In situ monitoring of
a 1 : 1 mixture of Pt(CH3)2(cod)/3a·HH in toluene/C6D6 revealed
the immediate formation of two new species at d(P) 24.8 ppm [1JPtP
1905 Hz] and d(P) 25.2 ppm [1JPtP 1831 Hz]. For both speices,
1
2
the magnitude of JPtP was indicative of k -P,N-chelation with
trans coordinated methyl ligands. We believe restricted rotation
about the amide bond leads to these two species which are E/Z-
conformational isomers.14a Over ca. 2 h the intensity of these
phosphorus signals diminishes as a new signal at d(P) appears
corresponding to the formation of 8a in solution. When 8b was
refluxed in toluene under N2 for 68h only the starting compound
was isolated, clearly indicating that a second methane protonation
by the benzylalcohol group is extremely slow. Likewise, 8a in
Scheme 8
resonances at d(P) 19.7 ppm (for 11c) and 22.7 ppm (for 11f)
are indicative of square-planar, diamagnetic species. Alternatively
11b could be prepared, in one-step, by reaction of Pd(OAc)2 and
3a·HH in CH2Cl2.
The X-ray structures of the Pt, Pd and Ni complexes 11a–11c
and 11d have each been determined (Fig.’s 7, 8 and ESI; Table 2).
1
refluxing toluene for 24 h showed no evidence, by 31P{ H} NMR
spectroscopy, of O-coordination despite the more acidic phenolic
4
In compounds 11a–11c tetradentate k -PNN¢O-complexation has
substituent.
clearly resulted with formation of one six- (M–P–C–C–C–N) and
two five-membered (M–N–C–C–N and M–N–C–C–O) chelate
rings. As an illustration, the three ring conformations in 11c are
described as follows: the Ni(1)–N(1)–C(7)–C(6)–C(1) part of the
3
Alternatively, k -PNN¢-tridentate ligation could be induced
by reacting 6a–6c, 7a or 7c with BuOK in CH3OH at room
t
temperature (Scheme 6). These clean transformations afforded
the compounds 9a–9c, 10a and 10c in good to excellent isolated
yields (51–96%). Very few examples of tridentate,28,32 and fewer
still of didentate,33 ligands have been shown to stabilise soft metal
˚
˚
ring is essentially flat (within 0.05 A) with P(1) lying 0.30 A out of
1
centres towards h -allyl coordination. Upon complexation, there
was a small change in d(P) of ca. 5–10 ppm for 9a–9c. However the
most significant insight was provided by new, sharp 1H resonances
1
for all five protons on the h -allyl ligand, clearly supporting
monohapto coordination. The solution stability of 10a towards
internal protonation by the OH group was also investigated since
Pd compounds are often more reactive than their Pt counterparts.
1
31P{ H} NMR monitoring of a CD2Cl2 solution of 10a over
several days at r.t. affored a mixture of phosphorus containing
compounds including 11b (vide infra).
Having established three distinct binding modes for these
ligands, we were interested to see whether they could function
4
as k -tetradentate ligands using all four donor atoms (PNN¢ and
O). Complexed ligands with either an ortho –OH or –CH2OH
group would be anticipated to use the O-donor in bonding to a
single metal centre with concomitant formation of a five- or six-
membered chelate ring. Accordingly reaction of 5a, 5b, bearing
the more acidic phenolic group, in CH3OH with tBuOK gave the
neutral compounds 11a and 11b. The nickel complex 11c was
obtained directly from NiCl2·6H2O, 3a·HH and tBuOK in CH3OH
(Scheme 7). Compounds 11d–11f were prepared cleanly using an
identical approach to those described for 11a–11c (Scheme 8).
Upon N,O-chelation, there is a marked reduction in 1JPtP for 11a
3
(3371 Hz) and 11d (3378 Hz) in comparison to the k -PNN¢-
tridentate compounds 8a (3865 Hz) and 8d (3834 Hz). Sharp 31
P
Fig. 7 Molecular structure of (a) 11c (compounds 11a and 11b are
isomorphous) (b) side-on view showing the ring conformations in 11c.
Scheme 7
7144 | Dalton Trans., 2010, 39, 7136–7146
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for the generous loan of precious metal salts and the EPSRC
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